std::make_shared
| Defined in header <memory>
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| template< class T, class... Args > shared_ptr<T> make_shared( Args&&... args ); |
(since C++11) | |
Constructs an object of type T and wraps it in a std::shared_ptr using args as the parameter list for the constructor of T.
The std::shared_ptr constructor called by this function enables shared_from_this with a pointer to the newly constructed object of type T.
Contents |
[edit] Parameters
| args | - | list of arguments with which an instance of T will be constructed.
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[edit] Return value
std::shared_ptr of an instance of type T.
[edit] Exceptions
May throw std::bad_alloc or any exception thrown by the constructor of T. If an exception is thrown, this function has no effect.
[edit] Notes
This function is typically used to replace the construction std::shared_ptr<T>(new T(args...)) of a shared pointer from the raw pointer returned by a call to new. In contrast to that expression, std::make_shared<T> typically allocates memory for the T object and for the std::shared_ptr's control block with a single memory allocation (this is a non-binding requirement in the Standard), where std::shared_ptr<T>(new T(args...)) performs at least two memory allocations.
Moreover, code such as f(std::shared_ptr<int>(new int(42)), g()) can cause a memory leak if g throws an exception because g() may be called after new int(42) and before the constructor of shared_ptr<int>. This doesn't occur in f(std::make_shared<int>(42), g()), since two function calls are never interleaved.
Although std::shared_ptr supports array types (as of C++17), std::make_shared does not. This functionality is supported by boost::make_shared
A constructor enables shared_from_this with a pointer ptr of type U* means that it determines if U has an unambiguous and accessible base class that is a specialization of std::enable_shared_from_this, and if so, the constructor evaluates the statement:
if (ptr != nullptr && ptr->weak_this.expired()) ptr->weak_this = std::shared_ptr<std::remove_cv_t<U>>(*this, const_cast<std::remove_cv_t<U>*>(ptr));
Where weak_this is the hidden mutable std::weak_ptr member of std::shared_from_this. The assignment to the weak_this member is not atomic and conflicts with any potentially concurrent access to the same object. This ensures that future calls to shared_from_this() would share ownership with the shared_ptr created by this raw pointer constructor.
The test ptr->weak_this.expired() in the exposition code above makes sure that weak_this is not reassigned if it already indicates an owner. This test is required as of C++17.
[edit] Example
#include <iostream> #include <memory> void foo(const std::shared_ptr<int>& i) { (*i)++; } int main() { auto sp = std::make_shared<int>(12); foo(sp); std::cout << *sp << std::endl; }
Output:
13
[edit] See also
constructs new shared_ptr (public member function) | |
| creates a shared pointer that manages a new object allocated using an allocator (function template) | |
| (C++14) |
creates a unique pointer that manages a new object (function template) |
